Tektronix OM4006D, User Manual, Page: 164 / 205

Appendix E: Equivalent-Time (ET) oscilloscope operation
Setting up an ET Oscilloscope
Equivalent-time
oscilloscope setup
See the following fi gure for how to connect the OM4000 instrument to take
measurements with equivalent-time (ET) oscilloscopes (Tektronix DSA8300
or DSA8200 sampling oscilloscopes).
Figure 21: Equivalent-time (ET) oscilloscope setup diagram
The most important difference between the real-time (RT) and equivalent-time
(ET) oscilloscope measurements is the need for a coherent reference signal for
ET oscilloscopes. The TX Reference signal is picked off before the modulator,
using a PM
fi ber cable, with a total path length equal to the path from the splitting
point to the Signal Input on the OM4000 Receiver. Use a SMF fi ber cable from
the DUT to the Signal Input connection on the OM4000.
Since the laser phase noise is a real-time quantity, it must be suf fi ciently
suppressed so that it can be tracked in the available bandwidth of the ET scope.
As an example, consider a laser with frequency noise given by
f(t) = f
0
+f
D
sin 2 π f
n
t
If this laser signal is split and then input to the Signal Input and Reference Input
of the OM4000, the resulting beat frequency will be
f
m
(t) ≈ (2 π f
D
f
n
∆ t) sin 2 π f
n
t
140 OM4000D Series Coherent Lightwave Signal Analyzer